One popular form of solid state image sensOr iS a CCD which converts a light signal (an image) incident upon a photosensitive input electrode thereof and generates at an output electrode thereof an electrical (video) signal corresponding to the light signal. A light sensing CCD typically comprises a p type semiconductor substrate with an insulating layer of silicon dioxide covering a maJor surface thereof and a plurality of spaced apart electrodes (gates) over the insulating layer. The electrodes are photosensitive and light incident thereon effectively generates charge carriers in a portion of the substrate therebelow. AdJacent electrodes are biased such that a potential well is created in the substrate under one of the electrodes but not under the other. Light incident on a pair of adJacent electrodes induces charge carriers in the substrate under both electrodes. The charge carriers under the electrode with the well thereunder are held in the well and the charge carriers under the other electrode are attracted toward and flow into the well Charge carriers are moved in the substrate from under one electrode to another by changing the relative biasing of adjacent electrodes. Typically complementary digital voltages provide the biasing which causes charge carriers to move in the substrate from under one electrode to under the next electrode. Thus the CCD acts as a shift register with light induced charged being transferred (shifted) from one electrode to subsequent electrodes and then to an output electrode where it is read out as an electrical (video) signal. One problem with this type of image sensor is that the digital voltages used to shift (transfer) charge from under one electrode to another introduces distortion and noise into the generated video signal which is undesirable.
Another popular form of solid state image sensor uses a photodiode coupled to a CCD. The CCD is essentially as described herein above except that it does not have a photosensitive electrode. This system also introduces noise and distortion due to the operation of the CCD.
U.S. Pat. No. 4,283,742 (M. Izumita et al), issued Aug. 11, 1981, describes and shows (see FIGS. 4 and 6) signal processing circuits in block diagram. Izumita et al teaches that these circuits are capable of limiting spike noise from a solid state imaging device 11 comprising an array of photodiodes with each photodiode being connected to the source drain circuitry of a separate MOS transistor. The circuit of FIG. 6 uses three sample and hold circuits 13, 14 and 22, a monostable multivibrator 18 , a pulse delay line 23 and a subtractor 17. Izumita et al is silent as to whether the components of his signal processing circuits of FIGS. 4 and 6 are on a common semiconductor substrate with the solid state imaging device 11 or if the imaging device 11 is on a separate semiconductor substrate. In either case the outputs of S/H circuits 22 and 14 of FIG. 6 couple the same noise and distortion components to the inputs of subtractor 17 thereby which effectively causing the same to be cancelled; however, the noise and distortion components also travel through the substrate and enter subtractor 17, whence and they are transmitted through subtractor 17 and appear at the output thereof.
It is desirable to be able to provide a high quality electrical representations of images from solid state imaging systems which use photosensitive CCDs or photodiodes coupled to conventional CCDs.